Boundary conditions for adsorbing-emitting electrodes in contact with seeded, dense plasmas

Adsorption and emission from boundary surfaces can profoundly alter the behavior of plasma-electrode systems, requiring a description that takes into account the physical characteristics of both the plasma and the surface. If the plasma is described in continuum terms, the surface properties enter through the specification of various species densities at the boundary, which strongly depend on the rates of emission of the respective species. Using the desorption rate expressions of Levine and Gyftopoulos for alkali metal coated metal surfaces, a method is proposed to determine boundary conditions for all species densities. The case of an equilibrium plasma confined between two plates is worked out in detail. The results suggest that ion densities could be very high near negatively biased boundaries, which in turn could lead to substantial field enhancement of thermionic electron emission. It is suggested that this mechanism might account for anomalously high currents observed in electrostatic probe studies.